This invention relates generally to lithography masks used for patterning layers of integrated circuits, and more specifically, to methods for making a stencil mask as can be used in conjunction with electron projection lithography, ion projection lithography, electron proximity lithography, and ion proximity lithography.
Electron projection lithography is a competing technology for future generations of lithography in semiconductor manufacturing (e.g. less than 70 nm feature sizes). Electron projection lithography projects electrons through a mask that has a pattern that correspond to the desired features to be patterned in a semiconductor wafer. In one form, the mask can have a continuous membrane through which the electrons pass, such as used in a technology known as SCALPEL(copyright) (SCattering Angular Limited Projection Electron Lithography). In another form, the mask membrane is discontinuous, having openings that correspond to desired features to be patterned in the semiconductor wafer. These are known as stencil masks. Stencil masks are also used for ion projection lithography that use ions as radiation source instead of electrons.
Existing stencil masks typically are formed from a silicon substrate, usually a silicon on insulator (SOI) substrate. Silicon is used as a support structure, and is also used to form a membrane layer that forms the stencil. The silicon membrane layer may be doped with boron or other dopant to control the stress in the membrane layer. A membrane window is formed by etching the silicon support structure, and the stencil pattern is formed by etching the silicon membrane layer. Typically, the silicon membrane layer has a thickness of approximately 1.5-3.0 xcfx80m when used in electron projection lithography applications. This thickness is required to provide a sufficient amount of electron scattering. In ion projection applications, the membrane layer is even thicker (approximately 1.5-10 xcfx80m). A thicker membrane layer is required in ion projection masks because the membrane window is usually much larger in area, thereby requiring more structural support.
There are two principal problems associated with existing stencil masks, both of which are related to the thickness requirements of the membrane layer. One problem is the ability to etch openings in the membrane layer that have high aspect ratios because it is difficult to control the sidewall profile of the opening through the membrane layer from top to bottom. The second problem is one known as image blur. As electrons or ions travel through a thick opening, some of the electrons or ions are drawn to the sidewalls of the opening due to interactions with the membrane materials. As a result, the electrons or ions passing through the opening are no longer as collimated as upon entering the mask opening. This causes the final projected image to be blurred in comparison to the desired pattern.
Materials other than silicon have been proposed for use as membrane layers. For example, U.S. Pat. No. 6,261,726 B1 by Brooks et al. proposes using a diamond, silicon carbide, diamond-like carbon, amorphous carbon, carbon nitride, or boron nitride to enhance the structural integrity of the membrane layer. However this patent requires a stencil membrane thickness of 0.5-5.0 xcfx80m, and therefore also suffers from the aforementioned problems.